The present invention relates to the art of optical integrated circuits and more particularly to apparatus and methods for mechanical beam steering for optical integrated circuits.
Optical integrated circuits (OICs) come in many forms such as 1xc3x97N optical splitters, optical switches, wavelength division multiplexers (WDMs), demultiplexers, optical add/drop multiplexers (OADMs), and the like. Such OICs are employed in constructing optical networks in which light signals are transmitted between optical devices for carrying data and other information. For instance, traditional signal exchanges within telecommunications networks and data communications networks using transmission of electrical signals via electrically conductive lines are being replaced with optical fibers and circuits through which optical (e.g., light) signals are transmitted. Such optical signals may carry data or other information through modulation techniques, for transmission of such information through an optical network. Optical circuits allow branching, coupling, switching, separating, multiplexing and demultiplexing of optical signals without intermediate transformation between optical and electrical media.
Such optical circuits include planar lightwave circuits (PLCs) having optical waveguides on flat substrates, which can be used for routing optical signals from one of a number of input optical fibers to any one of a number of output optical fibers or optical circuitry. PLCs make it possible to achieve higher densities, greater production volume and more diverse functions than are available with fiber components through employment of manufacturing techniques typically associated with the semiconductor industry. For instance, PLCs contain optical paths known as waveguides formed on a silicon wafer substrate using lithographic processing, wherein the waveguides are made from transmissive media including lithium niobate (LiNbO3) or other inorganic crystals, silica, glass, thermo-optic polymers, electro-optic polymers, and semiconductors such as indium phosphide (InP), which have a higher index of refraction than the chip substrate or the outlying cladding layers in order to guide light along the optical path. By using advanced photolithographic and other processes, PLCs are fashioned to integrate multiple components and functionalities into a single optical chip.
One important application of PLCs and OICs generally involves wavelength-division multiplexing (WDM) including dense wavelength-division multiplexing (DWDM). DWDM allows optical signals of different wavelengths, each carrying separate information, to be transmitted via a single optical channel or fiber in an optical network. For example, early systems provided four different wavelengths separated by 400 GHz, wherein each wavelength transferred data at 2.5 Gbits per second. Current multiplexed optical systems employ as many as 160 wavelengths on each optical fiber.
In order to provide advanced multiplexing and demultiplexing (e.g., DWDM) and other functions in such networks, arrayed-waveguide gratings (AWGs) have been developed in the form of PLCs. Existing AWGs can provide multiplexing or demultiplexing of up to 80 channels or wavelengths spaced as close as 50 GHz. As illustrated in FIG. 1, a conventional demultiplexing AWG 2 includes a base 4, such as a silicon substrate, with a single input port 6, and multiple output ports 8. Multiple wavelength light is received at the input port 6 (e.g., from an optical fiber in a network, not shown) and provided to an input lens 10 via an input optical path or waveguide 12 in the substrate base 4.
The input lens 10 spreads the multiple wavelength light into an array of waveguides 14, sometimes referred to as arrayed-waveguide grating arms. Each of the waveguides or arms 14 has a different optical path length from the input lens 10 to an output lens 16, resulting in a different phase tilt at the input to the lens 16 depending on wavelength. This phase tilt, in turn, affects how the light recombines in the output lens 16 through constructive interference. The lens 16 thus provides different wavelengths at the output ports 8 via individual output waveguides 18, whereby the AWG 2 can be employed in demultiplexing light signals entering the input port 6 into two or more demultiplexed signals at the output port 8. The AWG 2 can alternatively be used to multiplex light signals from the ports 8 into a multiplexed signal having two or more wavelength components at the port 6.
A problem with optical integrated circuits, such as the conventional AWG 2 of FIG. 1 is temperature sensitivity. Since the waveguide material usually has a temperature dependent refractive index, the channel wavelengths of multi/demultiplexer shift as the temperature varies. This shift is typically of the order of 0.01 nm/xc2x0 C. in silica-based devices and 0.1 nm/xc2x0 C. in InP based devices. This wavelength shift can result in a loss of signal and/or cross talk in communication system(s) employing the AWG 2. As communication system(s) are designed with increasingly smaller channel spacing, even a small temperature dependent wavelength shift can have a significant effect on system performance. Presently, AWG""s must have active stabilization of the device operating temperature in order to perform acceptably. This stabilization is typically achieved by the addition of resistive heaters, temperature sensors, active electronics, and in some cases also thermoelectric coolers. Even though an AWG is a passive filter, currently it requires significant electronics and a few watts of power to operate effectively.
Accordingly, there remains a need for better solutions to temperature sensitivity in optical integrated circuits such as AWGs, which avoid or mitigate the performance reductions associated with conventional optical integrated circuits and provide for mitigation of active temperature stabilization and its associated costs.
The following presents a simplified summary of the invention in order to provide a basic understanding of some aspects of the invention. This summary is not an extensive overview of the invention. It is intended to neither identify key or critical elements of the invention nor delineate the scope of the invention. Rather, the sole purpose of this summary is to present some concepts of the invention in a simplified form as a prelude to the more detailed description that is presented hereinafter.
The present invention provides optical integrated circuit apparatus and methods for mechanical beam steering mitigating and/or overcoming the shortcomings associated with conventional optical integrated circuit(s) and other devices. The invention further comprises methods for fabricating OICs and for mitigating temperature sensitivity utilizing mechanical beam steering in OICs.
According to an aspect of the present invention, OICs are provided in which a first region and a second region are formed. The first region and the second region each have at least one waveguide and are coupled by a lens. An expansion block having a coefficient of thermal expansion different than a coefficient of thermal expansion of the first region and the second region is located between the first region and the second region. The expansion block expands and/or contracts with temperature changes causing the first region and/or at least a portion of the lens to move with respect to the second region. Thus, wavelength shift associated with waveguide temperature dependent refractive index can be mitigated.
According to another aspect of the present invention, an optical circuit (e.g., AWG or PLC) is provided in which an input region, a grating region and an output region are formed. The input region, the grating region and the output region each have at least one waveguide. The input region and the grating region are coupled by an input lens. Likewise, the grating region and the output region are coupled by an output lens. A first expansion block is placed between the input region and the grating region; a second expansion block is placed between the grating region and the output region. The first expansion block and the second expansion block each have a coefficient of thermal expansion different than a coefficient of thermal expansion of the input region, the grating region and the output region The first expansion block and the second expansion block expand and/or contract with temperature changes causing the input region and/or at least a portion of the input lens to move with respect to the grating region and the output region and/or at least a portion of the output lens to move with respect to the grating region. Thus, wavelength shift associated with waveguide temperature dependent refractive index can be mitigated.
Another aspect of the invention provides a methodology for fabricating an optical integrated circuit. The method comprises providing a base, forming at least one waveguide in an input region, forming at least one waveguide in a grating region and forming an input lens coupling the at least one waveguide of the input region to the at least one waveguide of the grating region. The input region and the grating region are then scroll-diced from each other such that a remaining mechanical continuity is generally through the lens area. Thereafter, a first expansion block is placed in between the input region and the grating region.
Optionally, the base can further comprise an output region. An output lens is formed coupling the at least one waveguide of the grating region to the at least one waveguide of the output region. The output region and the grating region are then scroll-diced from each other around the output lens A second expansion block is then placed between the output region and the grating region.